1. Field of the Invention
This invention relates generally to a supply voltage monitoring device for automotive battery, and more specifically to a supply voltage monitoring device for automotive battery which produces the average voltage of the supply voltage of an automotive battery to control the supply voltage of the battery by the light emitting state or the non-emitting state of a light emitting diode controlled by the average voltage.
2. Description of the Prior Art
In recent years, an increasing number of d-c-powered electric refrigerators, for example, battery-powered electric refrigerators on board a car or a boat, etc. have been used. In these electric refrigerators, cooling is effected by taking heat away from the surroundings by means of refrigerant evaporated in the evaporator. The evaporated refrigerant is recovered and compressed in a compressor for recirculation. To drive the compressor, a vibration type motor, called swing motor, is often used. Since an alternating current is required to drive the vibration type motor, an inverter is used to convert the direct current of the battery into an alternating current. The output characteristic of the refrigerator with respect to the input voltage of the inverter, that is, the output voltage of the battery can be expressed as shown in FIG. 1, for example. In FIG. 1, (A) is a characteristic curve representing the relationship between the d-c input voltage and the cooling capability, (B) is a characteristic curve representing the relationship between the d-c input voltage and the a-c output voltage of the inverter, and (C) a characteristic curve representing the relationship between the d-c input voltage and the a-c output current of the inverter. As is evident from FIG. 1, the a-c output voltage and current of the inverter decrease as the d-c input voltage, that is, the output voltage of the battery drops. This results in a remarkable decrease in the compression capability of the compressor, that is, the cooling capability of the refrigerator. In other words, as the output voltage of the battery lowers, the compressor has to operate for a longer time, increasing the energy consumption of the battery.
When such an electric refrigerator is installed on a so-called leisure vehicle, the supply voltage of the battery, which is connected to various other on-board electrical equipment, is adversely affected by the operating condition of such electrical equipment other than the refrigerator. In addition, the distance between the battery and the electric refrigerator often requires long lead wires connecting them, and a voltage drop in those wires cannot be neglected. When the battery output voltage drops for these reasons, the d-c input voltage of the electric refrigerator also drops, deteriorating the cooling performance. In some extreme cases, the refrigerator apparently operates in good order despite the loss of its cooling capability.
A need exists, therefore, for a means for monitoring the supply voltage of the battery so as to check whether the cooling effect of the refrigerator is properly maintained or lowered. Devices for monitoring the terminal voltage of a battery, etc. using light emitting diodes have been employed for this purpose. However, the supply voltage of an automotive battery as described above often undergoes great changes due to variations in the battery load during running, and particularly sharp kick-like voltage variations at the start of the starter. In monitoring the supply voltage of an automotive battery as described above, therefore, it is necessary to produce the average voltage of the supply voltage without picking up such instantaneous kick-like voltage variations and to cause the light emitting diode to respond with the average voltage only.